Heat source for pyrolysis process

Abstract

Systems and methods are provided for using a reverse flow reactor (or another reactor with flows in opposing directions at different parts of a process cycle) for pyrolysis of hydrocarbons. The systems and methods can include a reactor that includes a combustion catalyst to initiate and/or maintain combustion within the reactor in a controlled manner during the heating and/or regeneration portion(s) of the reaction cycle. A fuel can also be used that has a greater resistance to auto-combustion, such as a fuel that is composed primarily of methane and/or other hydrocarbons. During operation, the temperature in at least an initial portion of the reactor can be maintained at a temperature so that auto-ignition of the auto-combustion resistant fuel injected during the heating step(s) is reduced or minimized. This can allow combustion to be initiated when the auto-combustion resistant fuel comes into contact with the catalyst. Additionally, the amount and positioning of the catalyst within the reactor can be controlled so that combustion of the fuel takes place over a substantially longer period of time than combustion during a conventional reactor heating step. Because the fuel is moving within the reactor during combustion, extending the combustion time results in a substantial expansion of the volume where combustion occurs. Optionally in combination with an improved reaction cycle, this can expand the portion of the reactor that is directly heated by combustion, allowing for an improved temperature distribution within the reactor during the pyrolysis step.

What is claimed is: 
     
       1. A reverse flow reactor for performing cyclic reactions with flows entering the reactor from substantially opposing directions comprising:
 a first zone comprising a primary end, a secondary end, and at least a portion of a reaction zone, the at least a portion of the reaction zone being proximate to the secondary end of the first zone, the reaction zone comprising a catalyst system; 
 a second zone comprising a first end and a second end, the second zone comprising a recuperation zone proximate to the first end, 
 wherein the first zone and the second zone are in fluid communication, the second end of the second zone being proximate to the secondary end of the first zone, 
 wherein the catalyst system comprises a first portion of combustion catalyst having a first combustion catalyst density and a second portion of combustion catalyst having a second combustion catalyst density, 
 wherein the second combustion catalyst density is greater than the first combustion catalyst density, and 
 wherein the first end of the second zone is closer to the first portion of combustion catalyst than the second portion of combustion catalyst. 
 
     
     
       2. The reverse flow reactor of  claim 1 , wherein the catalyst system further comprises an additional portion of combustion catalyst having a greater combustion catalyst density than the second portion of combustion catalyst, the first end of the second zone being closer to the second portion of combustion catalyst than the additional portion of combustion catalyst. 
     
     
       3. The reverse flow reactor of  claim 1 , wherein the catalyst system further comprises a third portion of combustion catalyst having a third combustion catalyst density, the second portion of combustion catalyst being located between the first portion of combustion catalyst and the third portion of combustion catalyst, the second portion of combustion catalyst having a greater combustion catalyst density than the third portion of combustion catalyst; and wherein the primary end of the first zone is closer to the third portion of combustion catalyst than the second portion of combustion catalyst. 
     
     
       4. The reverse flow reactor of  claim 1 , wherein the combustion catalyst comprises Mn, Y, Al, or a combination thereof. 
     
     
       5. The reverse flow reactor of  claim 1 , wherein (i) the combustion catalyst comprises a supported catalyst, (ii) the catalyst system comprises supported combustion catalyst particles and second particles having a lower activity than the supported combustion catalyst particles, or (iii) a combination thereof. 
     
     
       6. The reverse flow reactor of  claim 1 , wherein the second combustion catalyst density is greater than the first combustion catalyst density by 50% or more relative to the first combustion catalyst density. 
     
     
       7. The reverse flow reactor of  claim 1 , wherein the second zone comprises a second portion of the reaction zone. 
     
     
       8. A method for performing pyrolysis, comprising:
 exposing a fuel stream comprising fuel and oxygen in a reactor to a catalyst system comprising a catalyst density gradient of combustion catalyst under combustion conditions to form a flue gas and to heat one or more surfaces in a reaction zone to a surface temperature of 750° C. or more, the one or more surfaces in the reaction zone comprising a peak temperature of 1000° C. or less, the fuel comprising 90 vol % or more CH 4 , the fuel stream flowing in the reactor in a first flow direction; and 
 exposing a hydrocarbon-containing stream to the one or more surfaces in the reaction zone to pyrolyze at least a portion of the hydrocarbon-containing stream to form a pyrolyzed product, a flow direction of the hydrocarbon-containing stream comprising a second flow direction that is substantially opposite to the first flow direction, 
 wherein the reactor comprises a reverse flow reactor for performing cyclic reactions with flows entering the reactor from substantially opposing directions comprising: 
 a first zone comprising a primary end, a secondary end, and at least a portion of the reaction zone, the at least a portion of the reaction zone being proximate to the secondary end of the first zone, the reaction zone comprising the catalyst system; 
 a second zone comprising a first end and a second end, the second zone comprising a recuperation zone proximate to the first end, 
 wherein the first zone and the second zone are in fluid communication, the second end of the second zone being proximate to the secondary end of the first zone, 
 wherein the catalyst system comprises a first portion of combustion catalyst having a first combustion catalyst density and a second portion of combustion catalyst having a second combustion catalyst density, 
 wherein the second combustion catalyst density is greater than the first combustion catalyst density, and 
 wherein the first end of the second zone is closer to the first portion of combustion catalyst than the second portion of combustion catalyst. 
 
     
     
       9. The method of  claim 8 , wherein the second combustion catalyst density is greater than the first combustion catalyst density by 50% or more relative to the first combustion catalyst density. 
     
     
       10. The method of  claim 8 , wherein the combustion conditions comprise initially exposing the fuel stream to the combustion catalyst at a temperature of 720° C. or more. 
     
     
       11. The method of  claim 8 , wherein the exposing a fuel stream to the catalyst system and the exposing a hydrocarbon-containing stream to the one or more surfaces comprises a reaction cycle, the reaction cycle comprising flowing one or more purge flows through the reactor. 
     
     
       12. The method of  claim 11 , wherein the reaction cycle further comprises:
 exposing a second fuel stream comprising fuel and oxygen to the catalyst system under combustion conditions, the second flow stream comprising the second flow direction; and 
 exposing a second hydrocarbon-containing stream to the one or more surfaces in the reaction zone to form a second pyrolyzed product, the second hydrocarbon-containing stream comprising the first flow direction. 
 
     
     
       13. The method of  claim 12 , wherein the catalyst density gradient of combustion catalyst comprises at least a third portion of combustion catalyst having a third combustion catalyst density, the second combustion catalyst density being greater than the third combustion catalyst density, the second portion of combustion catalyst being downstream from the third portion of combustion catalyst relative to the second flow direction. 
     
     
       14. The method of  claim 1 , wherein the fuel comprises 95 vol % or more CH 4 . 
     
     
       15. The method of  claim 8 , wherein the fuel is exposed to a temperature of 750° C. or more for 30 milliseconds or less prior to exposing the fuel to the catalyst system. 
     
     
       16. The method of  claim 8 , wherein the pyrolyzed product comprises ethylene. 
     
     
       17. The method of  claim 8 , wherein the combustion catalyst comprises Mn, Y, Al, or a combination thereof. 
     
     
       18. The method of  claim 8 , wherein (i) the combustion catalyst comprises a supported catalyst, (ii) the catalyst system comprises supported combustion catalyst particles and second particles having a lower activity than the supported combustion catalyst particles, or (iii) a combination thereof. 
     
     
       19. The method of  claim 8 , wherein the flue gas comprises 0.01 vol % or less of CH 4 . 
     
     
       20. The method of  claim 8 , wherein the fuel stream comprises fuel and air, the air comprising 15 vol % or more O 2 , the amount of O 2  being greater than a stoichiometric amount for combustion of the fuel by 1.0 vol % to 99 vol %, and wherein the fuel and air comprise 90 vol % or more of the fuel stream.